The present invention relates to a nitride semiconductor device for use as a blue light-emitting diode, a blue-violet laser, a high-speed transistor, or the like and to a method for fabricating the same.
A Group III nitride semiconductor (BzAlxGa1-x-y-zInyN (where 0≦x≦1, 0≦y≦1, and 0≦z≦1 are satisfied)) (hereinafter simply referred to as a nitride semiconductor) is a semiconductor having an extremely large band gap energy. For example, GaN has a band gap energy as large as 3.4 eV (at a room temperature). For this reason, a nitride semiconductor is considered to be promising as a material capable of emitting visible light ranging widely from blue to ultraviolet.
A nitride semiconductor is also considered to be promising as a transistor material capable of high-temperature and high-output operation because it has a high electron speed in a high electric field. Thus far, it has been impossible to provide a nitride semiconductor having an excellent crystal property since a growth temperature for a nitride semiconductor is generally high and there is no substrate material that can be lattice-matched thereto.
Ever since the development of a technology which grows, by metal organic chemical vapor deposition (hereinafter referred to as MOCVD), a nitride semiconductor on a sapphire substrate with a low-temperature buffer layer interposed therebetween, however, the crystal property of a nitride semiconductor has been improved and a light-emitting diode or a semiconductor laser has been produced on a commercial basis.
In general, crystal defects such as lattice defects on the order of 1×109 cm−2 are present in a GaN crystal formed on a sapphire substrate as a factor causing the degradation of the characteristics and reliability of a light-emitting element or an electronic device. To solve the problem, a lower-defect technology using epitaxial lateral overgrowth (hereinafter referred to as ELO) has been developed. By epitaxially laterally overgrowing GaN on, e.g., a mask composed of an SiO2 film or the like, the density of crystal defects has been reduced to the level of 1×107 cm−2.
A description will be given herein below to a semiconductor device according to a first conventional embodiment with reference to FIG. 15. Specifically, the description will be given to a structure of a nitride semiconductor layer device formed by using the epitaxial lateral overgrowth technology mentioned above (see, e.g., O. H. Nam et. al., Applied Physics Letters 71 (1997) p.2638)
As shown in FIG. 15, a first GaN layer 3 is formed on a sapphire substrate 1 with a buffer layer 2 made of GaN interposed therebetween. An SiO2 film 4 patterned in stripes and having a thickness of 100 nm is formed on the first GaN layer 3. A second GaN layer 5 containing crystal defects at a low density is formed on the SiO2 film 4 by epitaxial lateral overgrowth (ELO) using MOCVD. Since the region of the second GaN layer 5 located on the SiO2 film 4 is formed by epitaxial lateral overgrowth (ELO), it has crystal defects at a reduced density.
A laser structure is formed on the low-defect region of the second GaN layer 5 which is formed on the SiO2 film 4. Specifically, an n-type clad layer 6, an active layer 7, a p-type clad layer 8, and a p-type contact layer 9 each made of a nitride semiconductor are formed and an insulating film 10 composed of, e.g., an SiO2 film and having an opening is formed on the p-type clad layer 8. A p-type electrode 11 is formed over the insulating film 10 and the portion of the p-type contact layer 9 exposed in the opening 10a of the insulating film 10, while an electrode pad 12 is formed on the p-type electrode 11. On the other hand, a metal electrode 13 is formed on the n-type clad layer 6. In FIG. 15, the vertical lines extending in the first GaN layer 3, the second GaN layer 5, and the n-type clad layer 6 indicate crystal defects such as lattice defects and the number of the vertical lines indicates the number of the crystal defects. For example, the crystal defects are reduced in the region overlying the SiO2 film 4 as described above and a smaller number of crystal defects are indicated by the vertical lines which are smaller in number than in the other region.
In terms of enhancing the performance of a nitride semiconductor device in addition to reducing defects described above, a technology which selectively oxidizes a surface of a GaN layer has also drawn attention as a second conventional embodiment (See, e.g., Japanese Laid-Open Patent Publication No. 2001-267555).
Specifically, a heat treatment is performed with respect to a GaN layer in an oxygen atmosphere by using an Si thin film or the like as a mask material to oxidize the surface of the GaN layer, followed by the removal of the mask material. If a field effect transistor is produced on the surface of the GaN layer thus formed, the oxide film formed on the surface of the GaN layer allows the isolation of the device and increases the breakdown voltage of the device. Such a selective oxidation technology is also applicable to current confinement in a semiconductor laser or the like and the wide application thereof is expected.
In the case of producing a nitride semiconductor laser device by using the lower-defect technology shown above in the first conventional embodiment, however, mask alignment is difficult since a waveguide structure with a width of, e.g., 2 μm should be formed in the low-crystal-defect region formed on the mask with a width of about 5 μm. To enlarge the low-crystal-defect region formed on the mask, it is necessary to increase the film thickness of the GaN layer formed on the mask. In that case, however, a crack is formed in the GaN layer due to the different thermal expansion coefficients thereof. This causes the problem of low characteristic reproducibility or low production yield in, e.g., a semiconductor laser device. To reduce crystal defects, the foregoing mask composed of the SiO2 film may be formed a plurality of times but the problem is encountered that the cost for device production using a photolithographic step is increased.
According to the selective oxidation technology in the second conventional embodiment, on the other hand, the roughened surface of the GaN layer causes the following problem in the steps subsequent to the surface oxidation, which is, e.g., low reproducibility in a photolithographic step, the risk of a broken line when a metal wire is formed on the roughened surface, or the like. In the case of applying the oxide film formed by using the selective oxidation technology to a metal-oxide-semiconductor (hereinafter referred to as MOS) field effect transistor, the thickness of the gate oxide film varies under an electrode so that the reproducibility of device characteristics is degraded.